Ultrasound pulse-echo imaging has become an important modality for medical diagnosis. Pulses of ultrasound energy are produced in a transducer and directed into a body. The energy is scattered from organ boundaries and other impedance discontinuities within the body; generating echoes which are detected with a transducer (which may be the same transducer used for transmission) to produce electrical signals which are then processed to form an image of the internal body structures. Most ultrasound pulse echo imaging systems of the prior art generate images from information which is extracted from the AM envelope of the echo signals. Such systems usually make use of a peak detector to extract a video signal from the echoes and generate a display by modulating the intensity of each pixel as a function of the amplitude of a corresponding portion of the video signal. Regions of the body which return strong (i.e.: high amplitude) echoes, for example organ boundaries, will thus be depicted as bright areas in the image whereas regions which return low amplitude echoes, for example homogeneous regions within the liver, will be depicted as darker areas in the image. This apparatus is more completely described, for example, in Medical Ultrasound Imaging: An Overview Of Principles And Instrumentation, J. F. Havlice and J. C. Taenzer; Proceedings of the IEEE, Vol. 67, No. 4, April 1979, pages 620-640, which is incorporated herein, by reference, as background material.
In B-scan imaging, an ultrasound transducer is translated and/or angulated along the surface of a body undergoing examination. A two-dimensional image is generated by plotting the detected characteristic of echoes at an image point which corresponds to the coordinates of the scatterer which produce the echoes. The depth coordinate of the scatterer is determined by measuring the time delay between pulse transmission and the receipt of the corresponding echo signal. The lateral coordinate of the scatterer is determined by measuring the lateral position and/or angulation of the transducer.
More recently, Dr. Leonard Ferrari has described a technique for producing images utilizing information which is contained in the FM envelope of an ultrasound pulse-echo signal (Dr. Ferrari's U.S. Pat. No. 4,543,826 is incorporated herein by reference as background material.) This technique maps the instantaneous phase or frequency of an echo signal into intensity levels in an image. For example, regions of the body which return echoes with higher instantaneous frequencies may be displayed as bright areas while regions of the body which return echoes with lower frequencies may be displayed as darker areas. A squelch circuit may further be provided which turns-off the FM detector and displays a neutral intensity level in the event that the amplitude of the echo signal is too low for FM detection.
Dr. Ferrari's patent application describes a system in which the intensity of regions in an image is normally independent of the amplitude of the echo signal. More recently, imaging systems have been developed wherein the intensity of pixels is modulated as a nonlinear function of both the amplitude and the frequency deviation of the echo signal (such systems are hereafter referred to as "Compound Systems").
Compound Systems appear to have the ability to delineate diseased regions within certain organs (for example, the liver) which were not heretofore discernable in systems which used either pure AM or pure FM detection. However, prior art Compound Systems suffered from low dynamic video range and, in some cases, when they were adjusted to display diseased areas within a particular organ (for example, the liver) they could not, without readjustment, clearly image other structures or organs.